Not sure how this is done really but I'm not really able to
change the shader on decals at all. Is it possible to alter the decals with my own shader. For instance creating animated blood decals?
I have created a sort of animated blood that is spreading along the plane.. What I would like to do is to have this shader act like a decal or mesh overlay.

you can try render mesh using default solid shader
and additional time using custom blend (blood) shader
just like in the custom shader tutorial / 12 - Flow Effect

maybe it would be better do call MeshPart::drawOverlay (instead of blend)
and specify custom shader for RM_OVERLAY

That RM_OVERLAY custom shaders sounds pretty cool but I also use the default overlays.